If you did the same, the simplest method is to extract the contents of the Xoops /html directory into C:\Program Files\xampp\htdocs\xoops.

Then when you browse to http://localhost/xoops/, I think the Xoops installer will automatically detect the correct path and URL, and place them in C:\Program Files\xampp\htdocs\xoops\mainfile.php. If not, you can provide those values manually:

However, I did this a bit differently. I edited C:\Program Files\xampp\apache\conf\httpd.conf and changed the location of the document root, because I wanted my htdocs directory to be in a different place.